The present invention relates generally to digital-to-analog (D/A) conversion systems and methods for generating an analog output from a digital signal, and more specifically to digital audio systems for generating an audio output from a digital signal. The present invention also relates to D/A conversion systems, in which the analog output can take the form of an electrical, acoustic, electromagnetic, thermal, liquid, gaseous, mechanical, or any other suitable type of analog output.
Audio source information has traditionally been accessed as analog signals and recorded in analog form, for example, on magnetic tape or phonograph records. There are two basic types of amplifiers that may be employed in the processing of analog signals. One type of amplifier is the conventional analog amplifier, which can be implemented in different topologies and classes depending upon how much current is allowed to flow through the amplifier's output transistors or tubes in the quiescent state, e.g., when the transistors or tubes are not delivering power to loudspeakers in an audio system. Such conventional analog amplifiers include the class A amplifier, the class B amplifier, etc. Another type of amplifier is the conventional switching type amplifier such as the class D amplifier, which has been employed for many years in various high efficiency industrial and medical applications. Like the class A and class B amplifiers, the class D amplifier may be implemented using transistors or tubes. However, instead of producing amplified analog signals that can have many different values, the class D amplifier switches between two voltage levels to produce a binary-level pulse width modulation (PWM) electrical output signal. As a result, at any given time, the output transistors or tubes of the class D amplifier are either “on” or “off”. In a typical audio application, the binary-level PWM output of the class D amplifier is low-pass filtered to suppress rapid changes in the output waveform, and the average value of the PWM output is employed to drive loudspeakers of an audio system.
Since the advent of the digital age, most audio source information has been accessed as digital signals and recorded in digital form using, e.g., CD or MP3 digital formats, and therefore a D/A conversion of an input signal in digital form to an output signal in analog form is generally required to enable the input signal to be amplified by conventional amplifiers driving loudspeakers of an audio system. More recently, there has been increased interest in developing audio systems that do not require the D/A conversion circuitry typically included in conventional audio systems. One such typical audio system is discussed in co-pending U.S. patent application Ser. No. 10/819,573 filed Apr. 7, 2004 entitled MULTI-LEVEL PULSE WIDTH MODULATION IN DIGITAL SYSTEM (the '573 application) by the same inventor as the present invention, which describes a typical PWM-based digital audio system that includes an interpolator stage, a noise shaper stage, a PWM converter stage, and a switching output stage. Like the conventional analog-input class D amplifier, the switching output stage of the system described in the '573 application produces a PWM output, which is typically filtered by a low-pass filter and used to drive one or more loudspeakers of an audio system.
Although the audio system described in the '573 application has been successfully employed for generating an audio output signal from a digital signal, the system has drawbacks in that the resolution of the system is limited by the switching speed of the power output switching transistors included in the switching output stage. To reduce the switching speed requirement of the switching output stage, a D/A conversion technique may be employed that uses a “coarse” signal and a “fine” signal to represent the digital signal. In this kind of D/A conversion technique, one unit of the coarse signal is equivalent to a fixed number of units of the fine signal. In other words, each unit of the coarse signal carries more weight than one unit of the fine signal. The coarse and fine signals are converted independently to some form of intermediate electrical signals, and then electrically combined with different weightings to produce the electrical output signal of the D/A conversion. Because the maximum number of units possible for the coarse and fine signals are both smaller than the maximum number of units possible for the digital signal, the resolution requirements for the D/A conversion of the coarse and fine signals to their respective intermediate electrical signals are lower than the resolution requirement for the direct D/A conversion of the digital signal.
Japan Patent No. 58087916 entitled DIGITAL-TO-ANALOG CONVERTER (the '916 patent) discloses a D/A converter that employs a D/A conversion technique that uses coarse and fine signals with different weightings to represent a digital signal. Specifically, the '916 patent discloses a technique that employs PWM signals having different amplitudes. The coarse signal comprises the most significant bits (MSBs) of the digital signal, and corresponds to a PWM signal having a larger amplitude to account for the higher weighting of the coarse signal. The fine signal comprises the least significant bits (LSBs) of the digital signal, and corresponds to a PWM signal having a smaller amplitude to account for the lower weighting of the fine signal. The two PWM signals having different amplitudes are combined electrically via a low-pass filter to produce the electrical output signal of the D/A conversion.
The system disclosed in the '573 application also employs coarse and fine signals like the D/A converter disclosed in the '916 patent, but in a significantly different way. Specifically, the system of the '573 application employs a D/A conversion technique to generate a multi-level PWM signal (i.e., PAM (pulse amplitude modulation)+PWM), from which the electrical output signal of the D/A conversion is produced. As disclosed in the '573 application, a coarse signal comprises the MSBs of a digital signal and corresponds to the multi-level component (i.e., the PAM component) of the multi-level PWM signal. Further, a fine signal comprises the LSBs of the digital signal and corresponds to the PWM component of the multi-level PWM signal. The PAM component is a multi-level fixed-pulse-width signal, while the PWM component is a single-level PWM signal. The multi-level PWM signal is formed by combining the single-level PWM signal with the multi-level fixed-pulse-width signal. The width of the multi-level PWM signal is equal to the width of the PAM component, which is equal to the maximum width of the PWM component plus one unit width. Each amplitude level of the PAM component is a multiple of the amplitude of the PWM component. Each amplitude level of the PAM component represents a value of the coarse signal and reflects the fact that the coarse signal carries more weight than the fine signal. The system disclosed in the '573 application includes switching circuitry for generating the multi-level PWM signal, which is filtered by a low-pass filter to produce the electrical output signal of the D/A conversion.
The '573 application discloses additional embodiments which enhance the multi-level PWM technique disclosed therein. For example, in one embodiment, the resolution requirement for resolving the coarse signal is reduced by having each one of a multiplicity of channels resolve a portion of the coarse signal at the same time instead of having a single channel resolve the entire coarse signal. The outputs of the multiple channels are additively combined to produce the equivalent output produced by a corresponding single channel system.
The '573 application also discloses a multi-channel digital audio system, in which an audio output signal is generated from a digital signal. A data sample of the digital signal is represented by a plurality of signal groups comprising a PWM signal corresponding to the LSBs of the digital signal (the fine signal) and multiple sets of control signals corresponding to the MSBs of the digital signal (the coarse signal). Each of the plurality of signal groups represents a partial value of the data sample of the digital signal. The sum of these partial values of the data sample is equal to the value of the data sample. In the multi-channel digital audio system of the '573 application, each signal group controls a corresponding switching output stage, in which each switching output stage is associated with a corresponding one of a plurality of channels. The signal group that controls the switching output stage of a fixed one of the plurality of channels comprises the PWM signal corresponding to the fine signal and one of the sets of control signals corresponding to a portion of the coarse signal. The switching output stages in the remaining channels are controlled by the remaining sets of control signals corresponding to the remaining portions of the coarse signal. Each signal group therefore controls a respective switching output stage of a respective channel to produce a respective electrical signal, which is filtered by a respective low-pass filter before being provided to a respective loudspeaker to produce a respective audio component signal. The electrical signal generated by a respective switching output stage can be an ordinary PWM signal, a PAM signal, or a multi-level PWM (i.e., PAM+PWM) signal. The respective audio component signals are then additively combined in the transmission medium, e.g., the air, to produce the audio output signal.
As described above, the '573 application discloses a digital system that employs a D/A conversion technique that uses coarse and fine signals to represent a digital signal, in which the fine signal is processed by a fixed one of multiple channels, and a portion of the coarse signal is processed by each of the multiple channels. Further, the system includes a plurality of switching output stages, in which each switching output stage is associated with a corresponding one of the plurality of channels. Because a respective portion of the coarse signal is processed by each switching output stage, it is possible to have a lower resolution requirement for each switching output stage to resolve the respective portion of the coarse signal than the resolution requirement needed by a single switching output stage for resolving the complete coarse signal. In other words, it is possible to have a smaller number of different voltage levels for the digital system because each switching output stage requires fewer voltage levels to represent a respective portion of the coarse signal, and the set of voltage levels associated with each switching output stage can be identical. Therefore, the number of different voltage levels minus one for the digital system as compared to the number of different voltage levels minus one for a corresponding single channel digital system can be reduced by a factor equal to the number of switching output stages.
It should be noted that the term “channel” is used herein in a generic sense, and is independent of other channelization that may occur in a digital audio system, such as traditional stereo channels, or surround sound channels, e.g., the left, center, right, rear left, rear right, etc., audio channels of a multi-channel audio system that accepts multi-channel digital signals as input, from which a respective digital signal is obtained as input for each respective audio channel of the system, in which each respective digital signal contains information independent of the other digital signals. In such a multi-channel audio system, each audio channel may contain multiple associated channels as explained above, and the audio output of a respective one of the audio channels may be produced by additively combining the audio component signals provided by the multiple channels associated therewith. Each of the audio channels produces information that is independent of the other audio channels, while each of the channels associated with a respective one of the audio channels provides a portion of the information produced by the respective audio channel.
One drawback of the above-described D/A conversion techniques that use coarse and fine signals with different weightings to represent a digital signal is that the coarse and fine signals must generally be converted with a high level of accuracy because any error in the signal conversion is magnified by the corresponding weight of the signal. For example, if a 16-bit data sample of the digital signal is divided into a fine signal comprising the 8 LSBs having a value of 100 and a coarse signal comprising the 8 MSBs having a value of 100, then the coarse signal weighs 256 times the fine signal. An error of, e.g., about 0.01% in the conversion of the coarse signal is therefore equivalent to a value of about 2.56% of the fine signal.
Another drawback of the above-described D/A conversion techniques result from the different responses to the coarse and fine signals that can be produced by the digital system. Specifically, when the digital signal varies from a value slightly less than a specified unit of the coarse signal to a value slightly greater than the specified value of the coarse signal, or vice versa, the coarse signal can increase by one unit while the fine signal changes from a maximum value to a minimum value, or vice versa. In an ideal system, such changes in the coarse and fine signals can offset each other and produce the desired analog output. However, due to the different responses to the coarse and fine signals produced by practical systems, such a slight variation in the value of the digital signal can cause unwanted transient outputs. This is particularly problematic for multi-channel systems like the one described in the '573 application because the fine signal is processed by a fixed one of multiple channels, while the coarse signal is processed by all of the channels. As a result, it is generally difficult to match or compensate the response of the fixed channel for the fine signal and the responses of all of the channels for the coarse signal to avoid occurrences of transient outputs, especially if the analog output is a non-electrical output.
One way of solving this problem of multi-channel systems is disclosed in co-pending U.S. patent application Ser. No. 11/103,952 filed Apr. 12, 2005 entitled SCHEMES TO IMPLEMENT MULTI-LEVEL PWM IN DIGITAL SYSTEM (the '952 application) by the same inventor as the present invention. As disclosed in the '952 application, a D/A conversion system has multiple channels, in which each channel can produce either a multi-level PWM signal like the system described in the '573 application, or two PWM signals like the system described in the '916 patent. The fine signal is processed by all of the channels in the system according to the value of the coarse signal, instead of being processed by a fixed one of the channels. Because the fine signal is processed by all of the channels, the system of the '952 application can achieve smoother transitions in the outputs of the various channels in response to variations of the digital signal, thereby reducing the occurrence of unwanted transients in the analog output. However, the D/A conversion technique disclosed in the '952 application is limited in that it is primarily for use in PWM-based systems.
It would therefore be desirable to have an improved D/A conversion system and method that can produce an analog output from a digital signal using D/A converters with lower resolution. Such a D/A conversion system would be implementable as a PWM-based system, a sigma delta D/A conversion system, or any other suitable type of D/A conversion system, while avoiding the drawbacks of the above-described conventional D/A conversion systems and techniques.